The Tengen RAMBO-1 is an ASICmapper, canonically designated as mapper 64. This mapper is basically Tengen's version of the MMC3, but with some extra features. The RAMBO-1 came as a 40-pin PDIP. A variant with different mirroring control is mapper 158.

Registers

The RAMBO-1 has four pairs of registers at $8000-$9FFF, $A000-$BFFF, $C000-$DFFF, and $E000-$FFFF - even addresses ($8000, $8002, etc.) select the low register and odd addresses ($8001, $8003, etc.) select the high register in each pair.

Mirroring ($A000-$BFFE, even)

IRQ registers

IRQ latch ($C000-$DFFE, even)

All eight bits of this register specifies the IRQ counter reload value. When the IRQ counter is zero (or a reload is requested through $C001), this value will be copied into the IRQ counter at the end of the current scanline.

IRQ mode select / reload ($C001-$DFFF, odd)

Writing to this register also clears the IRQ counter so that it will be reloaded at next clock, or the next scanline, depending on the selected mode. This also resets the prescaler in cycle mode, so the next clock will occur 4 cycles later.

IRQ acknowledge / disable ($E000-$FFFE, even)

Writing any value to this register will disable counter interrupts AND acknowledge any pending interrupts.

IRQ acknowledge / enable ($E001-$FFFF, odd)

Writing any value to this register will enable counter interrupts AND acknowledge any pending interrupts.

IRQ counter operation

There are two IRQ modes: PPU A12 mode (also known as scanline mode) and CPU cycle mode.

In scanline mode, the counter is clocked using a very similar method to that used by the MMC3 and follows the same restrictions. In comparison to the MMC3, the actual interrupt triggers slightly later. Specifically, it is delayed until M2 falls twice after the PPU A12 rise that would have triggered the MMC3 interrupt.

In CPU cycle mode, the counter is clocked every 4 CPU cycles. The actual interrupt triggers one M2 cycle later than one would naively expect.

Whichever IRQ mode is being used, the counter behaves the following way:

It's still unknown how the extra kick works. Klax still requires +1 to run perfectly.

Most emulators run Skull & Crossbones with a glitched scanline at the continue screen OR during the gameplay (scorebar).

Variants

Mapper 158, used for Alien Syndrome, has mirroring like mapper 118 (TLSROM), where CIRAM A10 is connected to CHR A17, and bit 7 of each CHR bank mapped into PPU $0000-$0FFF controls which page of CIRAM is used for the corresponding nametable in $2000-$2FFF.